1. Technical Field
The present invention relates to an illumination device, an image display device, and a projector and in particular, to a technique of an illumination device using a laser light source.
2. Related Art
In recent years, a technique of using a laser light source as an illumination device of a projector has been proposed. For example, a diffractive optical element may be used as an overlapping unit that makes laser beams overlap on an illuminated surface. The diffractive optical element performs shaping and enlargement of an illuminated region and makes the light intensity distribution in the illuminated region uniform by diffracting a laser beam which is coherent light. By realizing a plurality of functions with the diffractive optical element, the illumination device can be made with a small number of components. As a result, it becomes easy to make the illumination device small and to save the space. When zero-order light and diffracted light from the diffractive optical element are incident on the illuminated surface, only a part of the illuminated region may become bright because the zero-order light and the diffracted light overlap each other. Since only a part of the illuminated region is bright, it becomes difficult to obtain good light intensity distribution. In order to solve the problem, the zero-order light is made to travel to a position other than the illuminated surface. As a result, it becomes easy to make the light intensity distribution in the illuminated region uniform. For example, a technique of providing an illuminated surface at a position other than a position on an extending line of a beam line of a laser beam emitted from a light source unit is proposed in JP-A-2007-58148. Zero-order light emitted from a diffractive optical element travels on the extending line of the beam line of the laser beam emitted from the light source unit and moves to the position other than the illuminated surface.
In the technique disclosed in JP-A-2007-58148, a principal beam of diffracted light emitted from the diffractive optical element is largely inclined with respect to a perpendicular line of a reference plane on which the diffractive optical element is disposed. Advanced design technology is required for the diffractive optical element in order to realize shaping of an illuminated region and uniform light intensity distribution for diffracted light whose principal beam is largely inclined with respect to the perpendicular line of the reference plane. For this reason, a problem that shaping of the illuminated region and making the light intensity distribution uniform may be difficult occurs in the related art.